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Borrowings (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Short-term Debt [Line Items]    
Short-term borrowings, weighted average rate 0.35%us-gaap_ShortTermDebtWeightedAverageInterestRate 0.38%us-gaap_ShortTermDebtWeightedAverageInterestRate
Short-term borrowings $ 334,804us-gaap_ShortTermBorrowings $ 337,042us-gaap_ShortTermBorrowings
Long-term borrowings 0us-gaap_LongTermDebt 0us-gaap_LongTermDebt
Federal Home Loan Bank Advances [Member]    
Short-term Debt [Line Items]    
Short-term borrowings 295,300us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
298,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Federal Home Loan Bank Overnight Borrowings [Member]    
Short-term Debt [Line Items]    
Short-term borrowings 129,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= fisi_FederalHomeLoanBankOvernightBorrowingsMember
198,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= fisi_FederalHomeLoanBankOvernightBorrowingsMember
Federal Home Loan Borrowings Short Term Advances [Member]    
Short-term Debt [Line Items]    
Short-term borrowings $ 166,300us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= fisi_FederalHomeLoanBorrowingsShortTermAdvancesMember
$ 100,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= fisi_FederalHomeLoanBorrowingsShortTermAdvancesMember